1. Field of the Invention
This invention relates to thermoelectric devices and in particular to thermoelectric materials for such devices.
2. Description of the Related Art
Thermoelectric devices for cooling and heating and the generation of electricity have been known for many years; however, their use has not been cost competitive except for limited applications.
A good thermoelectric material is measured by its "figure of merit" or Z, defined as EQU Z=S.sup.2 /.rho.K
where S is the Seebeck coefficient, .rho. is the electrical resistivity, and K is the thermal conductivity. The Seebeck coefficient is further defined as the ratio of the open-circuit voltage to the temperature difference between the hot and cold junctions of a circuit exhibiting the Seebeck effect, or EQU S=V/(T.sub.h -T.sub.c).
Therefore, in searching for a good thermoelectric material, we look for materials with large values of S and low values of .rho. and K.
Thermoelectric materials currently in use today include the materials listed below with their figures of merit shown:
______________________________________ Thermoelectric Material Peak Zeta, Z (at temperature shown) ZT ______________________________________ Lead Telluride 1.8 .times. 10.sup.-3 /.degree. K. at 500.degree. K. 0.9 Bismuth Telluride 3.2 .times. 10.sup.-3 /.degree. K. at 300.degree. K. 1.0 Silicon germanium 0.8 .times. 10.sup.-3 /.degree. K. at 1100.degree. K. 0.9 ______________________________________
Workers in the thermoelectric field have been attempting to improve the figure of merit for the past 20-30 years with not much success. Most of the effort has been directed to reducing the lattice thermal conductivity (K) without adversely affecting the electric conductivity.
Applicants have been issued two United States Patents (U.S. Pat. Nos. 5,436,467 and 5,550,387), which are incorporated by reference herein. In those patents, Applicants disclosed a thermoelectric element having a very large number of very thin alternating layers of semiconductor material having the same crystalline structure. In a preferred embodiment, superlattice layers of SiGe with Si as barrier layers demonstrated figures of merit of more than six times better than bulk SiGe. These superlattice layers were grown on a Si substrate using a sputtering technique in an argon atmosphere.
Kapton.RTM. is a trademark of Dupont Corp. and is used to describe a well-known polyimide material. Films made of this material are also extensively used.
While the thermoelectric elements described in the above two patents represented a major advancement in thermoelectric technology. The prior art technology required for efficient use, the removal of the substrate on which the thin layers were laid down.
What is needed are better methods of preparing superlattice thermoelectric materials, elements and devices which do not require substrate removal.